linux-stable/Documentation/kbuild/Kconfig.recursion-issue-01
Luis R. Rodriguez 1c199f2878 kbuild: document recursive dependency limitation / resolution
Recursive dependency issues with kconfig are unavoidable due to
some limitations with kconfig, since these issues are recurring
provide a hint to the user how they can resolve these dependency
issues and also document why such limitation exists.

While at it also document a bit of future prospects of ways to
enhance Kconfig, including providing formal semantics and evaluation
of use of a SAT solver. If you're interested in this work or prospects
of it check out the kconfig-sat project wiki [0] and mailing list [1].

[0] http://kernelnewbies.org/KernelProjects/kconfig-sat
[1] https://groups.google.com/d/forum/kconfig-sat

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: James Bottomley <jbottomley@odin.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mate Soos <soos.mate@gmail.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Michal Marek <mmarek@suse.com>
2015-10-08 15:36:16 +02:00

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# Simple Kconfig recursive issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# Test with:
#
# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
#
# This Kconfig file has a simple recursive dependency issue. In order to
# understand why this recursive dependency issue occurs lets consider what
# Kconfig needs to address. We iterate over what Kconfig needs to address
# by stepping through the questions it needs to address sequentially.
#
# * What values are possible for CORE?
#
# CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values
# that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y',
# CORE must be 'y' too.
#
# * What influences CORE_BELL_A_ADVANCED ?
#
# As the name implies CORE_BELL_A_ADVANCED is an advanced feature of
# CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y'
# we know CORE_BELL_A_ADVANCED can be 'y' too.
#
# * What influences CORE_BELL_A ?
#
# CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A.
#
# But that is a problem, because this means that in order to determine
# what values are possible for CORE we ended up needing to address questions
# regarding possible values of CORE itself again. Answering the original
# question of what are the possible values of CORE would make the kconfig
# tools run in a loop. When this happens Kconfig exits and complains about
# the "recursive dependency detected" error.
#
# Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be
# obvious that an easy to solution to this problem should just be the removal
# of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
# since CORE_BELL_A depends on CORE. Recursive dependency issues are not always
# so trivial to resolve, we provide another example below of practical
# implications of this recursive issue where the solution is perhaps not so
# easy to understand. Note that matching semantics on the dependency on
# CORE also consist of a solution to this recursive problem.
mainmenu "Simple example to demo kconfig recursive dependency issue"
config CORE
tristate
config CORE_BELL_A
tristate
depends on CORE
config CORE_BELL_A_ADVANCED
tristate
depends on CORE_BELL_A
select CORE